Add missing IWYU message_loop.h includes.

components/sync/model_impl/passthrough_metadata_change_list_unittest.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/sync/model_impl/passthrough_metadata_change_list.h"
 
 #include <iterator>
 #include <map>
 #include <memory>
 #include <set>
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "components/sync/model/mock_model_type_store.h"
 #include "components/sync/protocol/entity_metadata.pb.h"
 #include "components/sync/protocol/model_type_state.pb.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace syncer {
 
 namespace {
 
@@ skipping 146 matching lines @@
   mcl()->ClearModelTypeState();
   mcl()->UpdateMetadata(kKey1, EntityMetadata());
   mcl()->ClearMetadata(kKey1);
   EXPECT_EQ(1u, global_metadata_vector().size());
   EXPECT_EQ(1, clear_global_metadata_count());
   EXPECT_EQ(1u, update_metadata_map().size());
   EXPECT_EQ(1u, clear_metadata_set().size());
 }
 
 }  // namespace syncer